Add exportvideo, an utility extracts the video data from a nuppelvideo
file and writes a YUV4MPEG stream which can be piped to mpeg2enc.
In other words, it exports the nuppelvideo .nuv format for encoding
(S)VCD/DVD compliant MPEGs with mjpegtools.
PR: ports/73417
Submitted by: Frank W. Josellis <frank@dynamical-systems.org>